1. Field of the Invention
The present invention relates to an index generating system, an information retrieval system, and an index generating method.
2. Related Background Art
Presently, there are huge volumes of information stored in the form of documents and others on the Internet (WEB) and users access desired information by a method such as keyword retrieval. The conventional search engines are generally configured to use a relation between a retrieval condition entered by a user and document information, such as word distributions in document groups stored, to extract documents meeting the condition, and to provide the extracted documents in a priority order to the user. Recently, many documents for portable terminals assumed to be browsed from portable terminals such as cell phones and PDAs are being stored on the Internet and there are increasing needs for retrieval of them. In the retrieval of documents for portable terminals, sizes of documents for portable terminals are small and the number of appearing words is small because of restrictions on the screen size and communication capacity. For this reason, there was a problem that direct application of the conventional retrieval methods based on word distributions resulted in a failure in achieving accurate document retrieval in response to a user request.
In contrast to it, the information retrieval method described in Japanese Patent No. 3598738 was to combine a document as a retrieval target with other documents linked thereto and having similar contents, to make a document set, and to extend a retrieval index so as to characterize the document as the retrieval target by words appearing in the document set. Since this information retrieval method used the retrieval index also including words not appearing in the document as the retrieval target, it was expected to enhance the possibility of extraction of documents fitting a retrieval keyword (also called a query word).